Various electric double layer capacitors are known heretofore. Japanese Patent Laid-open Publication (JP-A1) No. 11-283871 discloses an invention that focuses in particular on the strength of a metal collector foil for such electric double layer capacitors. It is stated in this publication that certain improvements in an electrode assembly have been proposed but the proposed electrode assembly is relatively weak in strength and hence is likely to be damaged during its manufacture or when it is laminated with a separator to form a capacitor. Taking this prior problem into consideration, the invention disclose in the aforesaid Japanese publication seeks to provide a metal collector foil for use in the electrode assembly and having a strength which is sufficient to withstand a rupture energy of at least 3.0 kg mm.
Based on the disclosure of JP-A1-11-283871, many sample electric double layer capacitors were produced by the present inventors for evaluation. The produced samples indicated that the metal collector foil was satisfactory in terms of strength, but due to the resistance value increasing beyond an allowable limit as the times goes on, the operation performance as a rechargeable battery deteriorated significantly. Through an investigation made on various factors, the present inventors have found that an oxide film produced on the surface of the metal collector foil affects the operation performance of the capacitor. This is because the oxide film is an insulator, and so the performance of the rechargeable battery deteriorates as the amount of oxide film increases.
A plain collector foil (i.e., a collector foil before being subjected to an etching process) has a flat and smooth surface and hence is likely to allow the occurrence of electrode separation when it is used for adhesive bonding with an electrode material. To deal with this problem, it has been an ordinary practice to etch the plain collector foil to form a dense network of microscopic channels or pits in surface area thereby to increase an increased bonding strength relative to the electrode material.
For convenience of manufacture, many etched collector foils are stored for a period of from several hours to several days rather than advanced to a subsequent processing operation. After etching, the foil surface is activated and hence actively reacts with oxygen in surrounding air. Thus, an oxide film produce on the surface area of the etched collector foil unavoidably grows up during the storage of the etched collector foil. An attempt to remove the oxide film just before the etched foil is subjected to a subsequent process (i.e., a bonding process in which an electrode material is attached by adhesion bonding to the surface the etched collector foil) may induce an additional cost, which increases the manufacturing cost of the capacitor.
Another finding through the afore-mentioned investigation is that chlorine remaining on the surface of the etched collector foil has a great influence on the age-related deterioration of the capacitor. The residual chlorine is caused by chlorine ions contained in an etching solution. Accordingly, the etched collector foil necessarily involves residual chlorine.
The residual chlorine is generally removed by washing. To improve the quality, the washing operation is repeated several times. Such repeated washing operation is, however, objectionable from the viewpoint of manufacturing cost.
Japanese Patent Laid-open Publication (JP-A1) No. 2001-176757 discloses the use of 99.99% pure aluminum foil as a collector foil in an electric double layer capacitor. The disclosed aluminum collector foil has a copper content blow 150 ppm. As is well known, in the manufacture of aluminum, bauxite ore is used as a starting material to produce alumina, which is then placed in an electrolytic furnace for purification to produce an aluminum ingot having an ordinary degree of purity in the range of from 90.0 to 99.85%. When needed, the aluminum ingot with ordinary purity is subjected to a secondary purification process in which the three-phase electrolyzing method or the segregation method is used to increase the purity of the aluminum ingot to 99.99% or higher. Such high purity aluminum is expensive, as it requires purification to be done repeatedly. The 99.99% pure aluminum foil disclosed in JP-A1-2001-176757 is also a high purity aluminum foil and hence expensive to manufacture. The use of the high purity aluminum foil in the manufacture of an electric double layer capacitor necessarily increases the manufacturing cost of the capacitor.